The present disclosure relates to crane control systems and more particularly to a Rated Capacity Limiter (RCL) of a crane with a non-symmetrical outrigger arrangement.
Mobile cranes typically include a carrier unit in the form of a transport chassis and a superstructure unit having a boom for lifting objects. The superstructure unit is typically rotatable upon the carrier unit. In transport the crane is supported by the carrier unit on its axles and tires.
When used for lifting operations, the crane should normally be stabilized to a greater degree than is possible while resting on the tires and axles of the transport chassis. In order to provide stability and support of the crane during lifting operations, it is well known to provide the carrier unit with an outrigger system. An outrigger system will normally include at least two (often four or more) telescoping outrigger beams with inverted jacks for supporting the crane when the crane is located in a position at which it will perform lifting tasks.
RCL systems have been developed to monitor the load the crane is lifting and alert the operator of unsafe operating conditions. Traditional RCL systems may be as simple as an indicator or audible alarm that sounds if a threshold is reached. For example, if the crane attempts to lift beyond a certain capacity, the alarm will sound. More recently, monitoring systems monitor the geometry of the crane and can alert the operator if the crane is moving into an unsafe operating condition. For example, a crane may have a constant load on the hook, but as it lowers the boom angle, the load moment increases. RCL systems may detect the change in boom angle and increase in load moment and alert the operator.
RCL systems typically have information referred to as load charts which indicate the maximum permissible load to lift depending on the crane configuration. One of the configuration characteristics is the positioning of the outriggers. Typically, there are four outriggers in a nearly square arrangement and the load charts only consider that the outriggers are extended from the vehicle at 0%, 50%, or 100%. Furthermore, the load charts assume that all the outriggers are extended to the same extent. Because the center-line of rotation is at approximately midway between the outriggers, the load chart can be assumed to be a “360 chart” since the minimum permissible load does not change with swing angle.
In some situations, a mobile crane may not be able to extend all of the outriggers to the same position. For example, a wall or other object may obstruct a single outrigger from extending, resulting in a non-symmetrical arrangement. The permissible load then becomes dependent on the swing angle. A cautious approach would be to select a load chart based on the minimum outrigger extension. This will provide a safe operating condition regardless of the swing angle. However, this load chart approach may restrict capacity of the crane that could be utilized. Alternatively, a load chart could be selected based on the position of the outriggers between the superstructure and the load. This would maximize the lifting capacity of the crane, but would require careful monitoring to ensure that the system did not do any lifting outside of a limited area.
It would be beneficial to develop a system that allows a mobile crane to perform lifting operations with a non-symmetric outrigger configuration. Furthermore, it would be beneficial if such a system did not unnecessarily limit the capacity of the crane or the swing angle of the superstructure.